These are temporary files that are created by the online fsck process when Splunk detects a bad index and tries to restore the buckets for that index. They are essentially, a working copy to use for restoring the index until the index has been restored to a good state. Once the -tmp folder has restored all the data for the working copy, it will then replace the same named folder. This is to ensure that data is not lost while the restoration is running.
Sometimes, these -tmp folders can be left behind if the online fsck process exits ungracefully. Generally, it should be safe to delete the folder as it is only a copy of the same named db_* folder in the same directory. The online fsck tool will simply re-create the -tmp folder if it is needed again.
More info on online fsck can be had here: http://wiki.splunk.com/Check_and_Repair_Metadata
